The US is sending about 200 extra troops to Iraq, along with helicopters and drone aircraft, after militants declared an Islamic state in areas under their control.
The latest deployment will mean nearly 800 US soldiers have returned to Iraq to help train local forces, protect the embassy in Baghdad and protect Americans as Islamic State in Iraq and Syria (ISIS) fighters continue to control much of the north and west of the country.
President Barack Obama has ruled out sending combat troops back to the country but has admitted the soldiers being sent will be equipped to fight.
ISIS fighters overran the Iraqi city of Mosul last month in a lightning offensive and have since been advancing towards Baghdad.
In Syria, they have captured territory in the north and east, along the frontier with Iraq.
The Sunni militant group has proclaimed its leader, Abu Bakr al Baghdadi, as “caliph” – the head of the state, for an area stretching from Aleppo in Syria to Diyala in Iraq.
Iraqi government forces have launched a counter-offensive in Saddam Hussein’shometown of Tikrit, about 100 miles north of Baghdad.
Tanks and artillery arrived on Monday to bolster the army on the edge of city.